A variety of medical devices for delivering a therapy and/or monitoring a physiological condition have been used clinically or proposed for clinical use in patients. Examples include medical devices that deliver therapy to and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other organs or tissue. Some therapies include the delivery of electrical signals, e.g., stimulation, to such organs or tissues. Some medical devices may employ one or more elongated electrical leads carrying electrodes for the delivery of therapeutic electrical signals to such organs or tissues, electrodes for sensing intrinsic electrical signals within the patient, which may be generated by such organs or tissue, and/or other sensors for sensing physiological parameters of a patient. Some medical devices may be “leadless” and include one or more electrodes on an outer housing of the medical device to deliver therapeutic electrical signals to organs or tissues and/or sense intrinsic electrical signals or physiological parameters of a patient.
Medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of therapeutic electrical signals or sensing. For example, electrodes or sensors may be carried at a distal portion of a lead. A proximal portion of the lead may be coupled to a medical device housing, which may contain circuitry such as signal generation and/or sensing circuitry. In some cases, the medical leads and the medical device housing are implantable within the patient, while in other cases percutaneous leads may be implanted and connected to a medical device housing outside of the patient. Medical devices with a housing configured for implantation within the patient may be referred to as implantable medical devices. Leadless medical devices are typically implantable medical devices positioned within or adjacent to organs or tissues within a patient for delivery of therapeutic electrical signals or sensing. In some example, leadless implantable medical devices may be anchored to a wall of an organ or to tissue via a fixation mechanism.
Implantable cardiac pacemakers or cardioverter-defibrillators, for example, provide therapeutic electrical signals to the heart, e.g., via electrodes carried by one or more medical leads or via electrodes on an outer housing of a leadless implantable medical device. The therapeutic electrical signals may include pulses for pacing, or shocks for cardioversion or defibrillation. In some cases, a medical device may sense intrinsic depolarizations of the heart, and control delivery of therapeutic signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate therapeutic electrical signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an implantable medical device may deliver pacing stimulation to the heart of the patient upon detecting tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting fibrillation.
In general, implantable medical devices require a small housing form factor to enable an unobtrusive implantation within a patient. In the case of leadless implantable medical devices, the housing form factor must be extremely small to enable implantation within or adjacent to organs or tissue. For example, a leadless pacemaker may be implanted directly into a ventricle of the heart. Battery usage is always a concern when designing implantable medical devices, but this concern is increased for small form factor devices that can only accommodate a small battery canister. A competing design requirement for implantable medical devices is high accuracy clocks that use a substantial amount of current. High clocking accuracy is needed to ensure accurate sensing and delivery of therapeutic electrical signals. Low-power clocks are too inaccurate due to poor long-term stability, temperature characteristics, and trim resolution to meet these requirements.